This proposal is responsive to PA-03-080 on "Biodefense and emerging infectious disease research opportunities". Our aim is to better understand how virulence is regulated in Staphylococcus aureus and to further develop immunotherapy to staphylococcal infections. S. aureus cause food poisoning, sepsis, device (Biofilm) related infections, and multiple diseases like endocarditis and pneumonia. The numerous toxins produced by the bacteria are at the heart of these diseases and cause more than 50,000 deaths each year in the US alone. As food and device-associated infectious organism that can survive in multiple environments, S. aureus is a potential target for bioterrorism. Many of the infective strains are resistant to conventional antibiotics and alternative methods to prevent and treat such infections are mandatory. We discovered that the production of toxins is regulated by quorum sensing mechanisms, where proteins such as RNAIIl activating protein (RAP) are secreted by the bacteria and induce virulence. RAP (native or recombinant) activates S. aureus pathogenesis through the histidine-phosphorylation of TRAP in a still unknown manner. TRAP has been shown to be membrane-associated, but it lacks a predicted transmembrane domain and a kinase domain typical of two component system sensors. Our first hypothesis is that TRAP is bound to the membrane through another protein and that this protein acts as the actual receptor to RAP. Studies proposed in Specific Aim 1 are focused on identifying this receptor. RAP has been shown to serve as an effective vaccine to prevent S. aureus infections. Our second hypothesis is that monoclonal anti RAP antibodies may be used for immunotherapy. This will be tested under Specific Aim 2, using a panel of clones already available. Such immune intervention would greatly benefit people suffering from acute drug resistant infections or surgery-related infections often associated with implantable medical devices.